1. Field of the Invention
The present invention relates to a display using cold cathode electron sources. More specifically, the present invention relates to a display suitable for an emissive type flat panel display using hot electron type electron sources.
2. Description of the Related Art
A display using cold cathode electron sources which are micro and can be integrated is called an FED (Field Emission Display). The cold cathode electron source is broadly divided into a field emission electron source and a hot electron type electron source. The former includes a spindt type electron source, a surface conduction type electron source and a carbon nano-tube type electron source. The latter includes an MIM (Metal-Insulator-Metal) type electron source stacked with metal-insulator-metal and an MIS (Metal-Insulator-Semiconductor) type electron source stacked with metal-insulator-semiconductor.
The MIM type electron source is disclosed in Japanese Patent Application Laid-Open No. 10-153979. The MIM type electron source will be described using FIGS. 1 and 2 schematically showing its structure and operating principle.
A driving voltage Vd is applied between a top electrode 13 and a bottom electrode 11 so that an electric field in an insulator 12 is about 1–10 MV/cm. Electrons near the Fermi level in the bottom electrode 11 pass through a potential barrier by tunneling phenomena and are implanted into a conduction band of the insulator (tunneling insulator) 12 and the top electrode 13 to be hot electrons. Of the hot electrons, ones which reach the surface of the electrode with an energy above a work function φ of the top electrode 13 are emitted into a vacuum 20. In FIG. 1, the numeral 14 denotes a protection insulator; the numeral 15, a top electrode bus line lower layer; the numeral 16, a top electrode bus line; and the numeral 17, an interlayer insulator.
When displaying an image in the FED, a driving method called a line sequential scanning scheme is used standardly. When displaying 60-frame still images per second, display in each of the frames is performed for each scan line (horizontally). All the cold cathode electron sources corresponding to the number of data lines on the same scan line are operated at the same time.
To the scan line at operation, is flowed an electric current obtained by multiplying an electric current consumed by the cold cathode electron source included in a sub pixel by the number of all the data lines and a color number 3 (RGB). The scan line electric current brings a voltage drop along the scan line by wire resistance to inhibit a uniform operation of the cold cathode electron source.
The voltage drop is different depending on the cold cathode electron source systems. In the Spindt type electron source as the field emission electron source, almost 100% of the electron source current is emitted into a vacuum to reach an anode (phosphor surface). An electric current flowed to a gate line (scan line) is very small so that the influence of the voltage drop is less.
In the surface conduction type electron source as the same field emission type and the MIM type and MIS type electron sources as the hot electron type, at most several % of an electron source current reaches the anode. Most of it is flowed as a reactive current into the gate line (scan line). With the same anode current, these electron sources are affected by the voltage drop more easily than the spindt type.
The present inventors have been involved in the study and development of the MIM type electron source. We have designed and prototyped several kinds of FEDs to examine image display. In the FEDs, the scan line has always been selected for the bottom electrode 11.
In the hot electron type electron source, the film thickness of the top electrode 13 must be very small as about several nm to reduce scattering of hot electrons. Since the sheet resistance is inevitably high as above 100 Ω/square, it is not suitable for the scan line.
The bottom electrode 11 is formed by an aluminum film having a film thickness of 300 nm. The scan line pitch is large as about three times the data line pitch. The line pitch is sufficient to easily suppress the sheet resistance to several 100 mΩ/square. It is very natural that the bottom electrode 11 is selected for the scan line.
It has been gradually apparent that this structure is difficult to suppress a significant voltage drop with increase in screen size.
In the FED, a scan line current Is required to obtain a predetermined brightness is expressed by the following equation (1):Is=Je×S/α  (1)
where Je: an anode current density to obtain a predetermined brightness, S: an area of a display screen, and α: a proportion of an anode current of an emitter current (also called an electron emission efficiency).
A voltage drop amount Vdrop produced at both ends of the scan line is expressed by the following equation (2):Vdrop=½×Id×Rs×(L/W)  (2)
where Id: a driving current, Rs: a sheet resistance of a scan line, L: a long side length of a display screen, and W: a line pitch of the scan line.
When assuming that the screen size is increased while maintaining a resolution constant, the voltage drop amount Vdrop is found to be increased in proportion to Rs×S/α.
To suppress this,
(1) The electron emission coefficient is increased.→The thickness of the top electrode 13 may be reduced. The lower limit is limited so that proportional reduction cannot be made.
(2) The sheet resistance Rs is lowered.→The thickness of the electrode is increased to reduce the resistivity. Improvement cannot be expected due to the following reasons (a) to (c).
(a) The tunneling insulator 12 must be of anodic oxidized alumina. Change of it to other materials cannot be made.
(b) Change of the deposition conditions (for example, making the substrate temperature higher) can lower the resistance of aluminum. The roughness of the film surface is deteriorated to impair the reliability of the tunneling insulator.
(c) When the film thickness is increased, the aluminum wire easily produces hillocks or voids in a heat treatment process. To prevent breakdown of the tunneling insulator, it is essential that the surface roughness of the electrode be maintained.
From the above views, in order that the MIM type electron source responds to a large screen display of a 40 inch class, it is essential to give a sheet resistance-scalable scan line.
To solve the above problems, an object of the present invention is to provide a display using hot electron type electron sources which can suppress a voltage drop amount produced in a scan line below an allowable range to obtain a high quality image without poor brightness uniformity when a screen size is increased.
To achieve the above object, the present inventors have variously experimented and studied such display using hot electron type electron sources to obtain the following findings. Using the top electrode bus line as the scan line and the bottom electrode as the data line, the MIM type electron source may display an image by the line sequential scanning scheme. In order to suppress the voltage drop amount Vdrop to an allowable range (for example, below 0.5V), the top electrode bus line may change the film thickness, the resistivity (material quality) and the deposition method to reduce the sheet resistance.
The present invention has been made based on such findings. The features of the present invention will be described in the following embodiments of the present invention.